In the semiconductor industry, silicides are generally used for contacting the semiconductor part of the transistor (i.e., the source/drain) and the poly-Si gate. In view of the requirement of low resistivity and low contact resistance, only three compounds selected from a list of possible metal silicide compounds are considered possible contact materials. The three best candidates are the C54-phase of titanium disilicide, cobalt disilicide and nickel monosilicide.
Problems with the nucleation of titanium disilicide in small dimensions prevent the use of titanium disilicide in advanced semiconductor devices. Hence, only cobalt disilicide and nickel monosilicide remain as viable candidates currently under consideration for contacting Si-based semiconductor substrates having narrow dimensions.
It is anticipated that SiGe or strained Si/SiGe may be used in the source/drain regions of advanced semiconductor devices in order to improve the electrical performance of the semiconductor device. Moreover, poly-SiGe can be used instead of the currently used poly-Si to contact the gate dielectric. Hence, there is an interest in finding a contact material for SiGe containing substrates.
It is known in the art that cobalt disilicide formation is delayed to higher temperatures in the presence of Ge atoms. The difficult nucleation of cobalt disilicide in the presence of Ge would greatly limit the choice of the contact material to only nickel monosilicide (either in its pure form, or in the presence of an alloying element) on SiGe containing substrates (“substrates” meaning either a single crystalline semiconductor in the source and drain regions, or a poly-SiGe layer on the gate of a device).
Because of the low resistivity and ease of formation, there would be a desire to provide cobalt disilicide contacts on SiGe containing substrates. In order for this to be a realization however, it is necessary to solve the problem of increased nucleation temperature which cobalt disilicide exhibits in the presence of Ge atoms.
FIGS. 1A-1C illustrate the difficulty of forming cobalt disilicide in the metallurgical presence of Ge in the reaction region. The vertical dotted lines in each figure highlight the disilicide formation temperature. The samples of these figures comprised 20 nm TiN, as a barrier layer, 8 nm Co containing x % Ge (x is 0 for FIG. 1A, 5 for FIG. 1B and 15 for FIG. 1C) and Si (100). The cobalt disilicide was formed by annealing each of the samples using an annealing ramp rate of 3° C./secs in purified He. In FIGS. 1B and 1C, the Ge was intentionally added to the reaction by depositing a Co—Ge alloy. Higher cobalt disilicide formation temperatures are required as the amount of Ge in the reaction region increases.
FIGS. 2A-2B illustrate the problem of forming cobalt disilicide on Ge containing poly-Si. The vertical dotted lines in each figure highlight the disilicide formation temperature. In FIG. 2A, n-doped poly-Si is employed, while in FIG. 2B p-doped poly-Si is employed. The disilicide formation temperature for the n-doped material is about 770° C. and for the p-doped material is 690° C. Especially, for n-doped poly-Si, the formation temperature is too high for standard processing recipes to work.
In view of classical nucleation theory, it may be expected that the nucleation of cobalt disilicide on a single crystalline SiGe containing substrate will be even more difficult. This is illustrated in FIGS. 3A-3E; these figures show the formation temperature of cobalt disilicide on different substrates. Specifically, FIG. 3A shows the cobalt disilicide formation temperature for 8 nm Co/silicon-on-insulator (SOI); FIG. 3B shows the cobalt disilicide formation temperature for 8 nm Co/30 nm Si/Si90Ge10/SOI; FIG. 3C shows the cobalt disilicide formation temperature for 8 nm Co/30 nm Si/Si80Ge20/SOI; FIG. 3D shows the cobalt disilicide formation temperature for 8 nm Co/30 nm Si/Si70Ge30/SOI; and FIG. 3D shows the cobalt disilicide formation temperature for 8 nm Co/30 nm Si/Si60Ge40/SOI. Clearly the higher the percentage of Ge in the substrate the higher the CoSi2 formation temperature hence the more difficult the nucleation.
In view of the above, there is a need for providing a method for forming cobalt disilicide contacts on top of a SiGe containing substrate which significantly reduces the silicide formation temperature to a value that is compatible with existing complementary metal oxide semiconductor (CMOS) processing schemes.